(a) Field of the Invention
The present invention relates to a color hard copy apparatus for printing on paper picture data of a picture displayed in a CRT frame connected to a color graphic terminal device. The color hard copy apparatus employs a simplified analog-to-binary data conversion method in order to simplify the video data processing circuits and also to reduce the cost of such color hard copy apparatus.
(b) Description of the Prior Art
Density pattern method and organized dither method are well known for obtaining a multicolor gradation picture on print output paper in a color hard copy system. According to the density pattern method, one picture cell is represented by a 2.times.2 matrix of subelements, 4.times.4 matrix of subelements or the like, and all the subelements forming picture cells are represented in either of two binary values (e.g. black or white, light or dark, print or unprint, etc.) to produce a hard copy of the picture. While, according to the organized dither method, a video signal of each picture cell is compared with one of the threshold levels by using an optimum threshold matrix called a dither matrix. Several embodiments for improving the quality of a picture are disclosed in Japanese Patent Laid-Open No. 52969/1984, etc. However, it is very difficult for conventional analog-to-binary data converting methods to effectively deal with video signals of very high frequency like 100 MHz or more being used in recent color graphic displays. In the case of a sequential comparison type analog-to-binary data conversion method, a threshold voltage is varied a number of times at each picture cell, and, therefore, this method can only respond generally to several MHz at best. Some parallel operation type analog-to-binary data conversion methods have a band width covering 100 MHz or above however, when the number of colors to be displayed increases, the number of comparators accordingly has to be increased and the data processing circuits become more complicated, which inevitably results in high cost of such apparatus.